Frame flex

term1te

I recently bought an Elite Turbo Muin, very impressed. It is very stable, and holds the bike much more rigidly than my old turbo did. However, I’ve now noticed how much the front of the frame moves or twists when pedalling, even when I’m not holding the handle bars. I guess it is only a few mm, which clearly you don’t see out on the road, or on a less ridged turbo. The frame is an 8 year old titanium VN Euros, which I’ve never thought of as being particularly flexy. I’d imagine most carbon frames would be more ridged, especially with a built up BB area.

How much energy is lost twisting the frame, and hence not moving you forwards? Everything else being equal, would you go noticeably faster with a significantly more ridged frame?

MountainMonster

Are you sure it's the frame twisting, and not the bike moving around in the front wheel holder? Or perhaps a slightly loose headset?

cougie

Some interesting stuff here - http://www.sheldonbrown.com/rinard/rina ... etest.html

You will get deflection - but I doubt its something mortals need worry about.

iron-clover

I have a simillar problem- when I have my alloy Trek in my elite turbo I can see the BB deflect noticably even under steady pedalling- it used to occur most when my left foot was pushing on the pedal and deflect to the right, possibly because the chainset acts as a slight stiffener?

It did turn out my left cleat had worn about 2-3mm shorter than the recently replaced right which didn't help at all and now movement is much reduced with a new cleat, but it is still there when putting the power down.

I have tried to replicate it out on the road but the bike just leans slightly instead- not enough to notice if you're not looking for it but it does move. I think it's just because the bike was not originally designed to be put in a turbo and those tubes not supposed to resist the flexing you put in.

homers_double

If your BB is moving then I'm assuming its a twisting force through the whole frame, don't forget you're only really clamped to the ground by two small sets of lugs.

sloppyschleckonds

Nothing to worry about, probably best not to look.

vinnymarsden

SloppySchleckonds wrote:

Nothing to worry about, probably best not to look.

I totally agree...
The BB area etc is forced to flex..the back wheel is suspended off the ground, and all your force is really going through that area, nothing directed ground wards…just watch Breaking Bad or something on laptop or man cave set up!!!!

veronese68

SloppySchleckonds wrote:

Nothing to worry about, best not to look.

FTFY, I don't think there is any probably about it.

homers_double

When the seat tube flexes, thats when you start to panic...

DKay

homers double wrote:

When the seat tube flexes, thats when you start to panic...

Unless it's a Trek Domane.

ai_1

The forces experienced by the frame in a turbo are not quite equivalent to those on the rod anyway. I'd expect a lot more flex on a turbo than the road since the frame is rigidly(ish) restrained via the the rear dropouts while a big mass (you) move around on top of it. It's not the same as when you're riding unrestrained and the bike is allowed to move beneath you.

keezx

Term1te wrote:

How much energy is lost twisting the frame, and hence not moving you forwards? Everything else being equal, would you go noticeably faster with a significantly more ridged frame?

Almost nothing.
Energy spent to bend the frame (or other parts) comes back in the system.

term1te

Keezx wrote:

Almost nothing.
Energy spent to bend the frame (or other parts) comes back in the system.

This is often said, and it is a long time since I studies A level physics, but I can't see how the energy that has been expended to bend the frame, even ever so slightly, can be returned to moving the bike forwards? Likewise, if the whole bike and rider move from side to side, how can the expended energy in moving the bike sideways be returned to rotate the crank or wheels? Won't the frame return to its original shape, and dissipate the spent energy as heat? I can see that by moving the bike side to side relative to your body whilst riding, you can use you upper body as well as your legs to rotate the cranks. I'm beginning to have visions of my old physics teacher now, so I better stop.

keezx

IF a force bends something on a bike,the part bending back will do the opposite, without asking energy from your muscles.
The efficieny depends on the elastic properties of the material, some materials will absorb more energy than others.
Extreme flexing will off coarse influence the efficiency of the human engine, but the difference between a flexy and stiff frame is negligable.

Bar Shaker

Energy wasted by flexing the bottom bracket, during the power section of the pedal stroke, cannot be returned in any way that will assist the propulsion of the bike.

In the same way, the energy expended in flexing wheels will not be returned as propulsion when climbing under high torque loads.

There is a reason why designers seek to make frames and wheels as stiff as possible. If you get onto a flexy bike, from riding your stiff modern carbon frame, you will feel like you are trying to sprint through mud.

keezx

Good point, it will feel different, the same way as light wheels will feel different.
Human feelings sometimes do not correspond with fysical facts.

Bar Shaker

Have you ever ridden a full suspension MTB and compared the road performance with the suspension open to that with it locked out?

keezx

No I've never ridden ANY MTB, when I finished my racing career the MTB was hardly invented.
Despite that, there is a good explanation for your observations....
A suspension is designed to have as little as possible elastic properties and to absorb energy.
You have to come with someting better, but i'm afraid if you come with a bad example it's the end....

homers_double

Energy expelled reading the last few posts will never be returned to my brain in any form, what a waste...

keezx

What is a waste for you isn't for everyone.....

Bar Shaker

It makes you wonder why we spend all that money on carbon and exotic alloys, when it seems that bikes and wheels made out of roto/injection moulded plastic would work just as well.

What fools we have been for coveting stiff components.

homers_double

Keezx wrote:

What is a waste for you isn't for everyone.....

I wonder if I could turn it into sarcasm.

keezx

Bar Shaker wrote:

What fools we have been for coveting stiff components.

Lots of fools out there indeed.....